1. Field of the Invention
The present invention relates to a thermosensitive recording medium that uses a coloring reaction between an electron-donating coloring compound and an electron-accepting compound, and the like, and that has a high image quality, glossiness, high density, and high tone, all of which are at the same level as those of silver halide photographic films, and thus that is particularly suitable for a medical image forming sheet for diagnosing or observing images of X-ray, MRI or CT.
2. Description of the Related Art
Thermosensitive recording media have been widely known in the art. Such thermosensitive recording medium has a thermosensitive recording layer provided on a support such as paper, and the thermosensitive recording layer contains an electron-donating coloring compound (also referred to as a coloring agent, hereinafter) and an electron-accepting compound (also referred to as a color developer, hereinafter). The thermosensitive recording medium utilizes a coloring reaction between the coloring agent and the color developer. This recording system has advantages such that a recording machine is compact and inexpensive, and maintenance thereof is easy. For this reasons, this recording system has been applied in the various fields, such as for facsimiles, ticket venders, printers for scientific measurements, printers for printing POS barcodes, and printers for CRT-based medical measurements.
In this recording medium, however, heat is directly applied from a thermal head to head a thermosensitive recording medium for recording. Therefore, various inventions have been made for a surface layer (generally called as an over layer or a protective layer) of the thermosensitive recording layer, which is brought into a direct contact with a thermal head. Various surface layers whose resin, lubricant, filler, and the like are developed have been proposed as inventions.
For using a thermosensitive recording medium in a medical working site, a surface of the thermosensitive recording medium is required to have high barrier properties to water or alcohol, as it is highly possible that the thermosensitive recording medium comes to in contact with water, alcohol or other solvents. It has been proposed that high barrier properties and glossiness are provided by using an electron-beam or ultraviolet curing resin, or water-soluble resin, core-shell emulsion containing a water-insoluble emulsion, and a crosslinking agent in combination.
However, in recent year, recordings of high speed and high accuracy have been demanded, and to meet the demand there has been a trend that electricity applied to each element of a thermal head is increased.
Moreover, a thermosensitive recording medium of medical use is used for diagnosis or references, and thus a subject for recording is mainly structural information or shape information, such as internal organs or bones of human bodies. It is important that the recognized image accurately reflects the original shape information, and images of deep blackness, high gradation, high glossiness, excellent concentration difference and contrast are expected.
Therefore, the higher gradation of images are required, and thus the higher output recording is required for thermosensitive recording media of medical use among all types of thermosensitive recording media including conventional leuco-type thermosensitive recording media. For such high output recording, a surface layer of a thermosensitive recording layer, having high heat resistance endurable with heat of a thermal head, as well as excellent glossiness, is desired.
Generally, to achieve head matching properties, it has been a common practice to increase proportions of filler and a lubricant to be added, as well as improving heat resistance of a resin.
However, use of the filler or lubricant reduces the surface glossiness of the resulting thermosensitive recording medium. Therefore, it has been proposed to reduce particle diameters of the filler or lubricant, or to reduce amounts of the filler or lubricant for use (see Japanese Patent Laid-Open (JP-A) No. 2008-73858, and Japanese Patent Application Publication (JP-B) No. 07-023025).
However, these methods reduce the heat resistance or lubricity of the film. The provided media by these proposal may have an effect for sticking, when they are used with high output thermal head in the recent medical recording, but sticking is only concerned when high energy is out put per image. The inventions proposed so far have not satisfies all of the various strict demands for practical use, such as prevention of depositions to a heating member occurred during continuous recording, high glossiness, water resistance, and solvent resistance. Therefore, further developments of thermosensitive recording media have been desired.
To reduce the amounts of the filler or lubricant, numbers of the inventions using, as a resin having extremely high heat resistance, a radiation-curing resin, such as an electron beam curing resin, and ultraviolet-curing resin have been made.
The electron beam curing resin is cured by irradiation of electron beams. Therefore, it is hardly affected by filler or additives, and desirable curing ability can be easily attained. However, it is difficult to control irradiation of electron beams because the thermosensitive recording layer may be colored by electron beams, or the support may be destroyed by electron beams. Moreover, there is also a problem that a large scale of a device is required (see JP-A No. 05-177943).
In contrast, in the case of ultraviolet irradiation, a device for use is simple, and ultraviolet rays do not destroy the support or color the thermosensitive recording layer, as long as cooling is performed to the support. However, there is a problem that yellowing of the thermosensitive recording layer occurs due to ultraviolet rays.
Particularly for the intended use in which excellent color tone of an image is required including whiteness of back ground and half-tone, yellowing of the back ground or half-tone is the problem to be solved.
Moreover, in the case where heat resistance of a radiation curing resin is attempted to be improved, curling of a thermosensitive recording medium tends to occur due to shrinkage on curing or distortion of the film. Since such thermosensitive recording medium is less flexible, problems, such as cracking, are caused by handling or under low temperature environment. Particularly, this is a problematic for the intended use in which the thermosensitive recording medium is handled in the form of a sheet (see JP-A No. 05-177943, Japanese Patent (JP-B) No. 3646815, and JP-A No. 07-81227). Note that, JP-B No. 3646815 discloses a reversible thermosensitive recording medium containing a reversible thermosensitive recording layer which is reversibly and repetitively changed from a transparent condition to a cloud condition by heating. The thermosensitive recording medium of the present invention concerns heat resistance thereof, and not the technology limited to thermosensitive (once) or reversible (rewrite), and therefore JP-B No. 3646815 is listed as a related art because it discloses the invention to solve the similar problem.